1. Field of the Invention
The present invention generally relates to film stress measurement systems and, more particularly, to systems that measure stresses in thin films, such as dielectric or metal alloy films that are coated onto semiconductor wafers.
2. State of the Art
During the manufacture of semiconductor wafers that serve as substrates for integrated circuits, it is usual practice to form thin films onto the surfaces of the wafers. The surface films can comprise, for example, silicon dioxide, AlSi, Ti, TiN, PECVD Oxide, PECVD Oxynitride, Doped Glasses, silicides, and so forth. The thickness of such films usually ranges from about 500 to about 12,000 Angstroms. Often, three or more film layers are formed on the surface of a single semiconductor wafer.
In the art of fabricating semiconductor wafers, it is of known importance to minimize stresses in surface films. High surface stresses can cause, for instance, silicide lifting, the formation of voids or cracks and other conditions that adversely affect semiconductor devices (i.e., chips) which are fabricated on the wafers. In practice, surface stresses become more problematical as the level of circuit integration increases, and are especially troublesome when fabricating large-scale integration (LSI) and very large-scale integration (VLSI) semiconductor devices.
Generally speaking, the stress in the surface film of a semiconductor wafer can either be compressive or tensile. In physical terms, stresses of both types can cause slight changes in the surface curvature of a semiconductor wafer. That is, both compressive and tensile stresses can cause the surface of a semiconductor wafer to deviate from exact planarity. The extent of deviation from planarity often is expressed in terms of the radius of curvature of a wafer surface. Generally speaking, the greater the magnitude of surface stress, the smaller the radius of curvature. Conversely, small stress magnitudes are manifest in terms of large radii of curvature.
Because of the problems that can be caused by stresses in surface films on semiconductor wafers, it is highly desireable to measure such stresses. The measurements can be used, for instance, to identify wafers that are likely to provide low yields of semiconductor devices or to produce semiconductor devices that are prone to early failure. In normal practice, stresses in surface films are not measured directly but, instead, are inferred from measurements of the radius of curvature of the surface of interest.
Mathematically, surface film stress can be related to the radius of curvature of a semiconductor wafer by a function which involves Young's modulus for the silicon substrate portion of the wafer, the Poisson ratio for the substrate, the thickness of the substrate, the film thickness, and the radius of curvature of the wafer due to surface film stress. As a matter of convention, the radius the radius of curvature is understood to be negative for convex curvatures and positive for concave curvatures. That is, negative values of a radius of curvature indicate compressive stress and positive values indicate tensile stress.
In the drawings attached hereto, FIGS. 1A through 1C show the effects of surface film stresses on the curvature of a semiconductor wafer 3. More particularly, FIG. 1A depicts semiconductor wafer 3 having a single surface film layer 5 which is under no stress. FIG. 1B depicts the same wafer under the condition where the surface film 5 exerts tensile stress, resulting in the wafer surface having concave curvature. FIG. 1C depicts the wafer with the surface film exerting compressive stress, resulting in the wafer having convex curvature.
The practical difficulties in measuring stress levels in surface films can be appreciated by noting that the radius of curvature of a semiconductor wafer is often measured in kilometers, or even hundreds of kilometers. By way of comparison, the diameters of semiconductor wafers are measured in inches, and the depths of surface layer films are measured in terms of a fraction of a micron.
A known system for making laboratory measurements of surface curvatures of semiconductor wafers is shown in FIG. 2. In the system, a beam of laser light is directed onto the surface of a semiconductor wafer and the reflected light is projected as a spot onto a screen. If the wafer surface deviates from exact planarity (i.e., has a radius of curvature), the location at which the reflected light strikes the screen will change as the wafer is moved perpendicularly to the beam (i.e., in the x-direction). Thus, by measuring both the distance that a wafer is moved and the resulting distance that the beam of reflected light moves on the screen, the wafer's radius of curvature can be determined.
In mathematical terms, the radius of curvature of a wafer can be related to measurements provided by the system of FIG. 2 as follows: EQU R=2L(dx/dd) (1)
where dx is the distance of translation of the wafer, dd is the resulting translation of the spot formed by the reflected beam on the screen, and L is the distance traveled by the reflected beam. In the system shown, the beam travel distance is about ten meters (i.e., L=10 meters). Further details concerning the system are provided in an article entitled "Thermal Stresses and Cracking Resistance of Dielectric Films on Si Substrates," A. K. Sinha et. al., Journal of Applied Physics, vol. 49, pp. 2423-2426, 1978.
Systems such as shown in FIG. 2 are often referred to as optically levered systems. Normally, calibration of such systems requires use of standard reference surfaces. Typically, one of the reference surfaces is assumed to be perfectly flat, and another of the reference surfaces has a predetermined radius of curvature.